1. Field of the Invention
The present invention relates to a discharge lamp and particularly to a structure of a discharge lamp suitable for use as a light source in lighting equipment such as a head lamp, a fog lamp and the like for an automotive vehicle.
2. Background Art
An embodiment of a structure of a conventional discharge lamp 90 for an automotive vehicle is shown in FIGS. 4 and 5. The discharge lamp 90 for an automotive vehicle is constituted by a discharge bulb 91, an outer cover tube 95 and a socket 96 for mounting the discharge lamp to a lighting device for an automotive vehicle (not shown). In this case, FIG. 4 shows a cross section in a vertical direction with the discharge lamp 90 mounted to the automotive vehicle (not shown), and FIG. 5 shows a cross section in a horizontal direction.
In this case, the discharge bulb 91 comprises a discharge chamber portion 91a and a seal portion 91b, as shown in FIG. 5. The discharge chamber portion 91a has a contour of a substantially spherical shape or a substantially cylindrical shape and is provided with a space for containing the discharge therewithin. The seal portion 91b is provided with discharge electrodes 92, a molybdenum foil 93 and a lead wire 94. The discharge electrodes 92 are held so as to oppose each other within the discharge chamber portion 91a with a suitable interval and electric power can be supplied to the discharge electrodes 92 from an outer portion.
Since the discharge bulb 91 is covered by the outer cover tube 95 (refer to FIG. 4), the discharge bulb 91 is prevented from being cooled by the open air and the like, which would reduce its efficiency. With the cover tube 95, it maintains a suitable temperature, thereby preventing a reduction its luminous efficiency. In most cases, a shield film 95a for shielding a portion which generates a harmful light such as a dazzling light and the like is provided when assembling the discharge lamp 90 to the automotive vehicle, on surface of the outer cover tube 95.
When the electric power is supplied to the discharge lamp 90 for the automotive vehicle having the above structure, the discharging is performed between the discharge electrodes 92, so that a noble gas such as xenon gas and the like and a metal halide such as scandium and the like sealed within the discharge chamber portion 91a emit light. Accordingly, for example, a white light having a color temperature near 4800.degree. K is emitted, and for example, which is excellent for color rendering in comparison with a light having a color temperature near 2800.degree. K, which can be obtained with a filament type lamp such as a halide electric lamp and the like.
However, in the conventional discharge lamp 90 mentioned above, since a non-evaporated metal halide H is mounted to a lower portion within the discharge chamber portion 91a extending over a relatively wide area (refer to FIG. 4), a light passing through the portion from an arc K, that is, a downward light, generates coloration and diffusion.
In this case, generally, since the lamp is vertically and laterally inverted after when being assembled in the lamp equipment for the automotive vehicle and its light is reflected in a reflecting mirror, the downward light passing through the non-evaporated metal halide mentioned above is directed upward light so as to cause an undesirable dazzling. In order to avoid this problem, a light distribution characteristic of the light equipment is generally made downward. However in this case, the light can not reach far, so that there is a problem that a distant visibility is reduced.
Further, secondly, when the seal portion 91b is formed in the conventional discharge lamp 90 for the automotive vehicle, only heating and pressing are performed and the shape of the seal portion 91b obtained thereby is not controlled, so that the curvature suddenly changes at a connecting portion 91c with respect to the discharge chamber portion 91a, so that a shining light spot L is generated near the connecting portion 91c, which gives the appearance that a plurality of light sources exist (refer to FIG. 5). Accordingly, there occurs a problem that it is difficult to form the light distribution characteristic that is needed in the light equipment for the automotive vehicle.
Still further, thirdly, the shape of the discharge chamber portion 91a is not sufficiently considered from an optical point of view. For example, in most cases, the thickness is suddenly changed at a portion near the seal portion 91b of the discharge chamber portion 91a, so that the light passing through the portion from the arc K receives an optical distortion such as a refraction and a change of shape. Accordingly, there occurs a problem that it is difficult to form the light distribution characteristic, as mentioned above. Accordingly, an object of the present invention is to solve the above problems.